Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions. For example, in a diesel engine, fuel is injected during the compression stroke, as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time to thoroughly mix the air and fuel before ignition occurs. The consequence is that diesel engine exhaust contains incompletely burned fuel known as particulate matter, or “soot”. In addition to particulate matter, internal combustion engines including diesel engines produce a number of combustion products including hydrocarbons (“HC”), carbon monoxide (“CO”), oxides of nitrogen (“NOx”), and oxides of sulfur (“SOx”).
After treatment systems may be utilized to reduce or eliminate emissions of these and other combustion products. For example, diesel particulate filters, such as catalyzed soot filters and others, can be used to trap diesel particulate matter and reduce emissions. The collection, or loading, of soot leads to an increase in exhaust pressure, which may degrade engine performance. To remove the particulate matter, the particulate filter can be passively regenerated by the presence of NO2 in the exhaust. Additionally, particulate filters may undergo active regeneration to eliminate trapped diesel particulate matter by adding external energy into the exhaust stream, thereby raising the temperature of the particulate filter to burn the soot that has accumulated therein. Active regeneration raises the temperature of the particulate filter up to approximately 400 degrees Celsius with a fuel-borne catalyst and up to 600 degrees Celsius without a fuel-borne catalyst.
The frequency of active regenerations impacts both the fuel efficiency of the vehicle and also the usable life of the after treatment system. Excessive regeneration of the particulate filter consumes additional fuel, thereby lowering fuel efficiency and raising operating costs. When too much time elapses between active regenerations, excessive soot builds up in the particulate filter. Because the burning of the soot is exothermic, a regeneration of a particulate filter with excessive soot results in even higher temperatures, which can increase the rate of wear on the after treatment system, thereby reducing the useful life of the system. Therefore, it may be advantageous to vary when a particulate filter is regenerated to reduce fuel consumption and extend after treatment system usable life.
Known methods for determining or estimating the soot load in a particulate filter suffer from various limitations. Thus, there is a need for one or more metrics for accurately and efficiently estimating particulate loading of particulate filters or soot filters in order to perform active regenerations that do not unduly impact fuel consumption or the effective life of the system.